Thermal phase sequences in gibbsite/kaolinite clay: electron microscopy studies

After heating the gibbsite/kaolinite clay to 200 °C or better at 300 °C, morphological characterization under a transmission electron microscope revealed that the chi-alumina from the gibbsite is identical in size and shape (hexagonal) to the undecomposed kaolinite crystals. The thermal transformations of gibbsite and well-crystallized kaolinite follow independent specific patterns up to 1100 or 1200 °C. Examining the organization of the internal granules formed from the dehydroxylation of gibbsite and kaolinite at increasing temperature (400-800 °C), a morphological difference was observed between the aluminas (chi and kappa) and the pseudomorphic metakaolin crystals. The two reaction sequences (a) metakaolin  spinel  mullite; (b) kappa-alumina  alpha-alumina occur in the 900-1100 °C range and characteristic morphological differences among the crystals are evident, especially in the spinel, which presents elongated particles. Mullite content increases between 1000 and 1550 °C, whereas alpha-alumina increases up to 1300 °C and decreases by 1550 °C, indicating interaction among the several high-temperature phases.